This invention relates to a drive mechanism for motor vehicles having two drive axles. More particularly, this invention relates to a drive mechanism having a differential and distributing gear unit following a transmission drive. The differential gear unit includes two rotatably driven members. Each of the two rotatable driven members drives one of the drive axles while frictionally coupled plates of a fluid friction clutch are associated with each of the driven members.
As is well known, a fluid friction clutch comprises sets of inner and outer plates which are connected to corresponding plate carriers. The plates and plate carriers are enclosed within a housing which is filled with a liquid having a high viscosity. The inner and outer plates, or their plate carriers, are coupled in rotation to each other by means of this liquid.
The two sets of elements of the fluid friction clutch are in turn connected and fixed in rotation to two output or driven members of a differential gearing unit. If there occurs a relative movement between the driven members of the differential gearing unit, for example, if one set of the wheels begins to slip, this relative movement is opposed by the fluid friction coupling with a torque which depends on the differential speed of rotation. This frictional torque then acts as a brake on the particular driven member of the differential gearing unit which is rotating at higher speed. The frictional torque arises from the shearing stress applied to the working fluid in the fluid frictional clutch when the driven members of the differential gearing unit drive the inner and outer sets of plates or elements at different speeds.
Self-locking differential gearing arrangements are already known and have proved themselves effective under actual working conditions. Hitherto, however, the fluid friction clutch unit has been integrated directly into the differential gearing units. Thus, for example, in one known embodiment, the outer plate carrier has been directly connected to the differential gearing housing, while the inner plate carrier has been directly connected to one of the output beveled shafts of the differential gearing unit. Consequently, the frictional torque provided by the fluid friction clutch unit has been of the same magnitude as that generated within the differential gearing unit itself. If only low rotational speeds are utilized within the differential gearing unit, it is necessary for the fluid friction clutch unit to be of a relatively large size in order to provide effective frictional torque. Moreover, in such known self-locking gearing units, it has been difficult to replace one frictional clutch unit with another having different characteristics in order to give the motor vehicle different driving characteristics because the fluid frictional clutch unit has been integrated into the differential gear unit.
British Patent No. 1,357,106, which is incorporated herein by reference, discloses a driving mechanism containing such a self-locking differential gearing unit for motor vehicles having two drive axles. The drive mechanism of this reference includes a fluid friction clutch which is integrated into a differential and distributing gearing unit. Thus, the two sets of frictional elements of the fluid friction clutch are directly connected to the driven members of the differential gear unit. As a result, this drive mechanism suffers from the disadvantage mentioned above.
In German patent document DE-PS No. 845,904, another drive mechanism for motor vehicles is disclosed. In this drive mechanism, an electromagnetic slip brake is coupled to a differential gearing unit and to the drive axle. A gearing system is located between the differential gearing unit and the electromagnetic slip brake. This gearing system provides a gear ratio greater than one in order to increase the rotational speed transmitted by the differential gearing unit. However, as is well known to those skilled in the art, a slip brake is completely different and has completely different characteristics from a fluid friction clutch.
It is therefore an object of the present invention to overcome the deficiencies mentioned above and to provide a drive mechanism for motor vehicles having two drive axles and a fluid friction clutch disposed between the differential gearing unit and the drive axles. It is also an object of the present invention to provide such a fluid friction clutch unit which is of a modular construction and is easily replaceable. It is also an object of the present invention to provide a fluid friction clutch unit which can be replaced by other fluid friction clutch units of different sizes and having different characteristics.